arXiv 1504.048122015-04astro-ph.EPastro-ph.GAastro-ph.SR

尘埃散射引起的原行星盘的毫米波偏振

Millimeter-wave polarization of protoplanetary disks due to dust scattering

Akimasa Kataoka, Takayuki Muto, Munetake Momose, Takashi Tsukagoshi, Misato Fukagawa, Hiroshi Shibai, Tomoyuki Hanawa, Koji Murakawa, Cornelis P. Dullemond

备注：: 18 pages, 20 figures, including the benchmark test of RADMC-3D, accepted for publication in ApJ
DOI：: 10.1088/0004-637X/809/1/78

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摘要：

提出了一种利用毫米波段偏振观测来约束原行星盘中晶粒尺寸的新方法。如果尘埃颗粒生长到与波长相当的尺寸，则尘埃颗粒预计具有大的散射不透明度，因此连续谱发射预计由于自散射而偏振。我们进行三维辐射转移计算，以估计偏振度的原行星盘具有径向高斯状尘埃表面密度分布，这是最近发现的。将最大粒径设定为$100 {\rm~μm}$，将观测波长设定为870 ${\rm μm}$。我们发现，偏振度是高达2.5%的subarcsec的空间分辨率，这是很可能被检测到与不久的未来ALMA观测。由于各向异性连续发射的散射，发射是偏振的。偏振度分布呈双峰分布，内环偏振方向为径向，外环偏振方向为方位角。我们还发现了偏振度的波长依赖性：当尘埃颗粒的最大尺寸为$a_{\rm max}\simλ/2π$时，偏振度最高，其中$λ$是观测波长。因此，对原行星盘的多波和空间分辨偏振观测使我们能够对晶粒尺寸施加约束。从偏振观测的晶粒尺寸的约束是独立的，甚至可能比从不透明度指数更强。

英文摘要：

We present a new method to constrain the grain size in protoplanetary disks with polarization observations at millimeter wavelengths. If dust grains are grown to the size comparable to the wavelengths, the dust grains are expected to have a large scattering opacity and thus the continuum emission is expected to be polarized due to self-scattering. We perform 3D radiative transfer calculations to estimate the polarization degree for the protoplanetary disks having radial Gaussian-like dust surface density distributions, which have been recently discovered. The maximum grain size is set to be $100 {\rm~μm}$ and the observing wavelength to be 870 ${\rm μm}$. We find that the polarization degree is as high as 2.5 % with a subarcsec spatial resolution, which is likely to be detected with near-future ALMA observations. The emission is polarized due to scattering of anisotropic continuum emission. The map of the polarization degree shows a double peaked distribution and the polarization vectors are in the radial direction in the inner ring and in the azimuthal direction in the outer ring. We also find the wavelength dependence of the polarization degree: the polarization degree is the highest if dust grains have a maximum size of $a_{\rm max}\simλ/2π$, where $λ$ is the observing wavelength. Hence, multi-wave and spatially resolved polarization observations toward protoplanetary disks enable us to put a constraint on the grain size. The constraint on the grain size from polarization observations is independent of or may be even stronger than that from the opacity index.

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